Processing semiconductor material



July 3, 1962 R. W. MYTTON ET AL PROCESSING SEMICONDUCTOR MATERIAL Original Filed Dec. 31, 1956 Roberr W. Myrron, PhyHis E. Stello,

INVENTORS.

A TTORNE Y.

United States Patent ware Original application Dec. 31, 1956, Ser. No. 631,844. Divided and this application July 18, 1969, Ser. No.

6 Claims. (Cl. 22-2165) This invention relates generally to processing semiconductor material and, more particularly,-to processing and casting the material in a specially prepared vessel which the material is caused to pass through, or is maintained in, in a molten state. This application is a division of application Serial No. 631,844, filed December 31, 1956, now abandoned.

In the prior art, it has been customary to use as a vessel for containing semiconductor material during processing thereof quartz or graphite crucibles, molds, boats, and the like. When the semiconductor material being processed was germanium, the particular vessel utilized was of relatively thick walled construction, and if con structed of quartz was coated with carbon. This allowed adequate and ready handling of the material and the vessel during all stages of processing.

It was, however, discovered that when certain semiconductor materials, for example,-silicon and aluminum antimonide, were being processed, they adhered to the vessel utilized to contain them during the processing. When this occurred, the semiconductor material, upon solidifying, cracked or shattered because of the difiference in coefficients of expansion between the material being processed and the material of which the vessel was constructed. This was particularly true of quartz vessels.

In order to combat this problem, particularly when silicon was the semiconductor material being processed, the quartz vessel was made to have exceedingly thin walls, on the order of 0.006 inch in thickness. This was done to keep the vessel from cracking the silicon material. The strains set up by the adherence of the materials and different coeflicients of expansion caused the vessel to crack before the silicon material could do so.

The thin walled quartz vessels, although being satisfactory for the purpose for which they were designed, created several problems. One problem which is obbious upon its face is that the quartz vessel is expended,

upon cooling of the silicon material, after being used only one time. Another problem is that, as above pointed out, quartz reacts with silicon and adheres to silicon during the processing thereof. Therefore, even if the vessel did not crack or shatter, it became an integral and bonded part of the silicon semiconductor material being processed.

Since it is not desirable to have quartz adhered to the silicon semiconductor material when it is to be utilized in semiconductor translating devices, the quartz had to be etched from the silicon semiconductor material. This necessitated an additional step in processing the material and added to the expense of making semiconductor translating devices of silicon, as well as destroying some of the silicon material before it could be used in any manner.

Graphite vessels were found to be unsuccessful for the processing of certain semiconductor materials such as silicon because it reacts with the silicon material during processing, thus forming silicon carbide.

The problems above pointed out become acute in the processing of silicon, high silicon content silicon-gen manium alloy, and certain of the intermetallics such as, for example, alurninum-antimonide. The above-referredto problems become acute when any type processing is 3,041,696 Patented July 3', 1962 "Ice utilized. Some of the types of processing currently used are the zone refining process, the well-known Czochralslq method of drawing crystals from a melt, or forming or processing of polycrystalline semiconductor materials.

Accordingly, it is an object of the present invention to provide for semiconductor materials processing in a vessel to which molten semiconductor material will not adhere, and which reduces contamination of the semiconductor materials during processing.

Another object of the present invention is to provide for processing in a vessel silicon which allows the formation of silicon single crystals by zone melting.

Still another object of the present invention is to provide for processing silicon semiconductor material in a crucible which allows the purification of silicon material to form substantially pure silicon single crystals by utilizing the zone melting process.

A still further object of the present invention is to provide for processing semiconductor materials in a Vessel that is not expended after one use thereof.

' A still further object of the present invention is to provide a crucible which does not adhere to the semiconductor material during processing thereof, thus eliminating the necessity of etching said material after the processing thereof to remove the adhered crucible therefrom.

In accordance with the present invention, there is provided a vessel having a configuration adapted to contain molten semiconductor material during the processing thereof. That portion of the surface of the vessel which is contacted by the semiconductor material is coated with a layer of silicon nitride. The silicon nitride coating prohibits the adherence of the semiconductor material to the side Walls of the vessel during processing, and thus solves the problems above-referred to.

The novel features which are believed to be characteristic of the present invention, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which:

FIG. 1 is an isometric drawing of a boat which may be utilized in carrying out the zone melting process;

FIG. 2 is a cross section of the boat shown in FIG. 1;

FIG. 3 is an isometric drawing of a crucible which may be utilized in carrying out the Czochralski method of crystal drawing; and

FIG. 4 is a cross section of the crucible of FIG. 3.

Referring now to the drawing and, more particularly, to FIG. 1 thereof, there is shown a boat 11 which may be formed of any desirable material which may be utilized in the processing of semiconductor materials. ticular configuration of the boat as shown in FIG 1 is readily adapted for utilization as part of the apparatus necessary to carry out the zone melting process. This process is fully set forth and described in US. Patent No. 2,739,088, entitled Process for Controlling Solute Segregation by Zone-Melting, issued March 20, 1956, to W. G. Pfann. While this boat may be constructed of variou materials, it has been found that for utilization in processing silicon, high silicon content silicon-germaniurn alloy, aluminum-antimonide semiconductor materials,

the flocked inner surface 12 of the boat 11. According to the preferred embodiment of the present invention, the

The par state.

silicon nitride coating should be substantially in powdered form.

The silicon nitridecoating may be applied to the inner surface of boat 11 in any manner which is desired. It

has been found, however, that the following method of; application of the silicon nitride is best adapted for purposes of this invention. The inner surface ofboat 11 is first sandblasted. The sandblasting performs two-functions: v(l) the entire inner surface of boat 11 is cleaned of any foreign matter which may be clinging thereto; 2) the innersurface of boat 11 is roughened, thus aiding the adherence of the silicon nitride powder thereto. Boat 11 is then heated to a temperature above the evaporating point of water. The exact temperature is not critical so long as the water or other liquid which is to be placed thereon will quickly evaporate ,upon contact. This will be more fully explained hereinafter. Powdered silicon nitride in substantially pure form i next mixed with a carrier which, in accordance with the preferred method of this invention, is plain tap water, to'

form a thin slurry. The percentage by volume of silicon nitride to water may be betweenthe limits of and percent. p

The thin slurry of silicon nitride is then sprayed evenly by. any convenient means upon the inner surface of heated boat'll. Upon contacting the inner surface of the heated boat, the water carrier immediately evaporates,

leaving the powdered silicon nitride which adheres to the inner surface of boat 11. Boat 11 is then allowed to cool to room temperature. I 7

Boat 11 now has its inner surface coated with a layer 12'of powdered silicon nitride and is ready for utilization as part of a zone melting apparatus.

"While the present invention may be used with either single or polycrystalline silicon, high silicon content germanium-silicon alloy and certain of the intermetallics, the remainder of the description will be taken with respect to siliconmaterial for purposes of example only.

The silicon nitride coating as applied in powdered form to the inner surface of boat 11 prohibits the silicon semiconductor material from adhering to boat 11 when the material is passed through, or maintained in, a molten Since the silicon material does not bond directly to the vessel, the differences in coeflicients of expansion between the vessel and thesilicon material do not cause cracking or shattering of either the vessel or the material. The contacting surfaces are allowed to slip or slide past each other. Upon completion of the processing of the silicon material, it may be readily removed from the boat 7 and the boat then re-used for further processing of the semiconductor material. It has been found that after utilization for the processing of two or three groups of material, the silicon nitride coating should be replaced.

Referring now more particularly to FIGS. 3 and 4,,

there is shown a crucible which may be utilized in the Czochralski method of semiconductor material processing. The silicon nitride coating 22 is applied in the same manner as above referred to and as above pointed out prohibits the material from adhering to the inner surface of crucible 21 during the processing thereof.

Although a boat and a crucible for utilization in the zone melting and Czochralski methods of processing, respectively, have been shown in the accompanying drawing, it is to be expressly understood that a vessel having any configuration for utilization in the processing of semiconductor material by any method may be utilized in carrying out this invention, It is to be further expressly UIIdGI'e stood that this invention is as readily applicable to the processing of polycrystalline semiconductor materials a it is to the processing of single crystal material.

There has been thus disclosed a method utilizing a vessel having a silicon nitride coating over the inner surface thereof, preferably applied in a slurry in a volatile liquid and dried, which prevents the adherence of certain semiconductor materials to the vessel during the processing thereof, thus providing reusable containers for the semiconductor material, and allows the formation or purification of single crystals of semiconductor materials, as well as the processing of polycrystalline semiconductor materials.

, What is claimed is:

l. The method of processing semiconductor material of the class of semiconductor materials which react with or adhere to quartz, which comprises: spraying the inner surface of a quartz crucible having a clean, roughened inner surface with a slurry consisting essentially of substantially pure powdered silicon nitride in a vaporizable liquid carrier; vaporizing said carrier to form an adherent coating consisting essentially of said powdered silicon nitride on said inner surface; and casting said semiconductor material in said crucible.

2. The method of processing semiconductor material of said powdered silicon nitride on'said inner surface;

and casting said semiconductor material in said crucible. 3. The method of processing silicon semiconductor material, which comprises: sandblasting the inner surface of a crucible to clean and roughen the same and thereby improve the adherence of crystalline powder thereto; spraying the inner surface of the crucible with a slurry consist ing essentially of substantially pure powdered silicon nitride in a vaporizable liquid carrier; vaporizing said liquid carrier; and casting said silicon semiconductor mater'al in said crucible. V

4. The method of processing aluminum antimonide semiconductor material, which comprises: sandblasting the inner surface of a crucible to clean and roughen the same and thereby improve the adherence of crystalline powder thereto; spraying the inner surface of the crucible with a slurry consisting essentially of substantially pure powdered silicon nitride in a vaporizable liquid carrier; vaporizing said liquid carrier; and casting said aluminum antimonide semiconductor material in said crucible.

5. The method of preparing a crucible for casting semiconductor material therein, which comprises: roughening the inner surface of the crucible;.and coating said surface with a layer of substantially pure powdered silicon nitride by forming a slurry which consists essentially of silicon nitride in 'avaporizable carrier, spraying a coat of said slurry on said surface, and-vaporizing said carrier.

6. The method of processing semiconductor material of the class of semiconductor materials which react with or adhere to quartz, which comp-rises; sandblasting the inner surface of a quartz crucible to clean and roughen the same; coating said sandblasted surface with substantially pure powdered silicon nitride; and casting said semiconductor material in said crucible.

References Cited in the file of this patent UNITED STATES PATENTS 2,201,049 Moore May 14, 1940 2,609,318 Swentzel u Sept. 2, 1952 V FOREIGN PATENTS 745,037 Great Britain Feb, 15, 1956 

1. THE METHOD OF PROCESSING SEMICONDUCTOR MATERIAL OF THE CLASS OF SEMICONDUCTOR MATERIALS WHICH REACT WITH OR ADHERE TO QUARTZ, WHICH COMPRISES: SPRAYING THE INNER SURFACE OF A QUARTZ CRUCIBLE HAVING A CLEAN, ROUGHENED INNER SURFACE WITH A SLURRY CONSISTING ESSENTIALLY OF SUBSTANTIALLY PURE POWDERED SILICON NITRIDE IN A VAPORIZABLE LIQUID CARRIER; VAPORIZING SAID CARRIER TO FORM AN ADHERENT COATING CONSISTING ESSENTIALLY OF SAID POWDERED SILICON NITRIDE ON SAID INNER SURFACE; AND CASTING SAID SEMICONDUCTOR MATERIAL IN SAID CRUCIBLE. 